1. Field of the Invention
The present invention relates to a yoke of a universal joint used in an automobile steering column and a method of producing the yoke.
2. Description of the Prior Art
An automobile steering column transmits the rotation of the steering wheel to the steering gearbox. For increasing the degree of freedom of geometric arrangement of the steering system, the steering column has a plurality of steering shafts which are connected with each other by universal joints. In general, the steering column is bent at each universal joint. With this, the steering wheel can be disposed at a desired position, and the steering column can be disposed in the engine room so as not to interfere with other parts disposed in the engine room.
In order to clarify the task of the present invention, a universal joint having a pair of conventional yokes and a cross will be outlined in the following with reference to FIGS. 13-17 of the accompanying drawings.
As is seen from FIG. 13, a universal joint 10 comprises a pair of yokes 12 and a cross 14. Each yoke 12 which is substantially U-shaped has a base portion 16 and a pair of arm portions 18. The base portion 16 is substantially parabolic in shape and has a circular opening 20 for connecting a steering shaft (not shown) with the yoke 12. The arm portions 18 are opposed to each other in a diametral direction of the yoke 12 and extend in an axial direction of the yoke 12. Each arm portion 18 has a circular opening 22 into which a bearing 24 is pressed. The cross 14 is rotatably held by the four bearings 24.
With reference to FIGS. 14-17, a conventional method of producing the yoke 12 will be described in the following.
At first, blanks 26 are formed by punching a long narrow rectangular steel plate 28 having a uniform thickness. Upon this, two punching manners are available with respect to the blank arrangement on the steel plate 28. One is that the blanks 26 are arranged in a manner to direct longitudinal axes of the blanks 26 in a direction which is perpendicular to a longitudinal axis of the steel plate 28 (see FIG. 14). The other is that the blanks 26 are arranged in a manner to direct longitudinal axes of the blanks 26 in a direction which is parallel to a longitudinal axis of the steel plate 28 (see FIG. 15). As is seen from FIG. 16, each blank 26 is elongate in shape and uniform in thickness and has first and second end portions 30, 32 and peripheral bulge portions 34 at its middle portion. After the blank formation, the blank 26 is pressed with a press machine so as to form the yoke 12 having the objective shape and a uniform thickness (see FIG. 17). With this pressing, in fact, the first and second end portions 30, 32 and the peripheral bulge portions 34 of the blank 26 are turned into the arm portions 18 and part of the base portion 16 of the yoke 12, respectively.
As is seen from FIG. 13, the universal joint 10 is produced by combining the yokes 12 with the cross 14 in a conventional manner. However, the thus produced conventional yoke has the following drawbacks.
The steel plate 28 can not efficiently be used by taking either of the above-mentioned two punching manners. In fact, oblique line portions shown in FIGS. 14 and 15 become waste due to the provision of the peripheral bulge portions 34 of the blank 26. This lowers the yield of the blank 26 and thus increases the production cost. As is seen from FIG. 16, fiber flows 36 (flows of metallographic structure) of the blank 26 are arranged in a longitudinal direction of the blank, and fiber flows 36 in the bulge portions 34 are sheared at their peripheral edges. Therefore, the blank 26 is lowered in strength. The arm portions 18 of the yoke 12 must have a certain thickness for holding the bearings 24 in the openings 22. Since the yoke 12 is uniform in thickness, the thickness of the base portion 16 becomes the same as that of the arm portions 18. Therefore, the yoke 12 becomes too heavy in weight.